1. Field of the Invention
Disclosed herein is a driving device for a unit pixel of an organic light emitting display, and more specifically, to a driving device for a unit pixel of an OLED having an improved structure in which the driving device can be more easily manufactured.
2. Description of the Related Art
Liquid crystal displays (LCDs), because they are not heavy and have low power consumption, are desirable for use in flat panel displays (FPDs). However, since LCDs are light-receiving type displays, it can be difficult to meet technical specifications for brightness, contrast, view angle, as well as use in larger sized displays. Development for new FPDs that can overcome these disadvantages has proceeded rapidly. Organic light emitting displays (OLEDs), a new possibility for use with FPDs, have a two-dimensional arrangement and self light-emitting pixels. Thus, OLEDs have excellent characteristics, such as excellent view angle and contrast. It is highly desirable that OLEDs do not need backlights, as are needed with LCDs. Thus, OLEDs can be made even lighter and smaller and have very low power consumption. In addition, since OLEDs can be directly driven by low voltages, have a fast response speed, and are entirely solid bodies, the OLEDs tend to be resistant to external shocks, and can be used in a wide temperature range. Also highly desirable is that the OLEDs have low manufacturing costs. Specifically, if OLEDs are driven using an active matrix method by which transistors as switching devices are disposed in each pixel, the OLEDs show the same brightness even if low currents are applied to the OLEDs. Thus, the OLEDs require less power, have high definition, and can utilized in large FPDs.
Two thin film transistors (TFTs), a switching TFT and a driving TFT, can be provided in each unit pixel of the active matrix type OLEDs, so as to drive an OLED. The switching TFT turns on/off a current supplied to each unit pixel, and the driving TFT provides currents allocated to the OLED. Due to this role assignment, characteristics and specifications of the switching TFT and driving TFT are different. For example, the switching TFT can have a small leakage current and a large on/off ratio. On the other hand, the driving TFT is required to have high mobility and reliability. In order to meet the requirements, the switching TFT can be formed as an amorphous silicon TFT and the driving TFT can be formed as a polycrystalline silicon transistor.
However, the process of manufacturing the switching TFT and the process of manufacturing the driving TFT are separately performed. This results in increased manufacturing complexity, which increases costs. Meanwhile, when an amorphous silicon TFT is simultaneously used as the switching TFT and the driving TFT, the amorphous silicon TFT is worthy of a switching TFT, but low electron mobility and reliability hinder practical applicability. Thus, it is difficult to implement an OLED having excellent characteristics. Contrary to this, when a polycrystalline silicon TFT is used as a switching TFT, the polycrystalline silicon TFT is worthy of a driving TFT, but a large leakage current in the off-state hinders practical applicability. Thus, the possibility of use of the polycrystalline silicon TFT as a switching transistor is reduced.